1. Field of the Invention
The present invention relates to an aqueous ink (hereinafter also referred to as ink), an inkjet printing method using the aqueous ink, a recorded matter obtained by the printing method, and devices using such an ink. More particularly, the present invention relates to a technique for making practical a surfactant which cannot be used or can be used only in a very small amount in an ink under normal conditions due to its low HLB (a surfactant which is very poor in solubility in an ink, and is separated and precipitated in a state in which an ink containing the surfactant is stored).
2. Related Background Art
Various studies and reports have been made for inks for use in writing implements and inks for inkjet recording for the purpose of improving image quality of the obtained recorded matter and fixation and reliability of the ink. For example, as methods for improving fixation of a recorded matter, a method in which water-soluble alcohol is incorporated in an aqueous ink, a method in which a highly hydrophobic water-soluble organic solvent, e.g., cellosolve is incorporated in an aqueous ink, and the like have been proposed. However, incorporation of the above compounds in an ink in a large amount improves fixation but degrades quality of a recorded matter. Moreover, use of the above compounds may cause problems as to inflammability of an ink and safety of human bodies.
On the other hand, a method in which a water-soluble surfactant is incorporated in an ink to improve fixation of a recorded matter is proposed in Japanese Patent Application Laid-Open No. H5-132643. It can be said that this method is preferable in terms of safety, but the viscosity of an ink increases as the content of the surfactant in the ink is increased for improving fixation of the recorded matter. For example, if the method is used for an inkjet printing method, reliability including ink ejection is impaired. If the surfactant is added in an ink in a large amount for further improvement of fixation, the ink may spread on the surface of a recording medium to degrade image quality of the obtained recorded matter.
On the other hand, it is proposed in Japanese Patent Application Laid-Open No. H6-256696 and Japanese Patent Application Laid-Open No. 2003-253599 that a hydrophilic surfactant is used in conjunction with a highly hydrophobic surfactant to improve fixation of a recorded matter. However, the amount of such a highly hydrophobic surfactant capable of being used in an ink is very small. Thus, fixation cannot be considerably improved compared to the conventional method using a water-soluble surfactant.
In Japanese Patent Application Laid-Open No. H11-71543, Japanese Patent Application Laid-Open No. H8-183921, Japanese Patent Application Laid-Open No. H6-9911, Japanese Patent Application Laid-Open No. H6-220376, Japanese Patent Application Laid-Open No. H10-140058, Japanese Patent Application Laid-Open No. H5-125313, Japanese Patent Application Laid-Open No. H10-265727, Japanese Patent Application Laid-Open No. H9-279067, Japanese Patent Application Laid-Open No. H9-217031, Japanese Patent Application Laid-Open No. H6-256696, Japanese Patent Application Laid-Open No. H10-130551 and Japanese Patent Application Laid-Open No. 2001-262026, various propositions are made for the method in which a hydrophobic compound is used in an ink. Among them, in Japanese Patent Application Laid-Open No. H11-71543, it is proposed that a water-insoluble material such as water-insoluble ethylene oxide, propylene oxide oil, water-insoluble alcohol or water-insoluble aldehyde is dissolved using an acid such as benzoic acid or xylene sulfonic acid, salt thereof, resorcin or the like, and using a co-solvent such as 2-pyrrolidone or glycol ester as required. However, in the above proposition, a water-insoluble material is dissolved using an acid or salt thereof, ester compound or polar compound, and therefore undesirable effects are produced on safety of human bodies, and solutes of internal additives and the like emerge in a member contacting an ink from the member to produce undesirable effects on stability and reliability of the ink. Moreover, if the acid or salt thereof is used in a large amount, precipitation easily occurs in the ink, or around an ink eject hole in the case of the inkjet recording method, for example, and thus reliability of the ink or in recording is impaired. Especially in the inkjet recording method, the influence may be significant because the size of ejected ink droplets is small.